In modem wireless communication systems, especially in CDMA (Code Division Multiple Access) wireless communication systems, in order to increase system capacity, system sensitivity and communication distances with lower emission power, smart antennas are generally used.
The Chinese patent named “Time Division Duplex Synchronous Code Division Multiple Access Wireless Communication System with Smart Antenna” (CN 97 1 04039.7) discloses a base station structure for a wireless communication system with smart antennas. The base station includes an antenna array consisting of one or more antenna units, corresponding radio frequency feeder cables and a set of coherent radio frequency transceivers. Each antenna unit receives signals from user terminals. The antenna units direct the space characteristic vectors and directions of arrival (DOA) of the signals to a baseband processor. The processor then implements receiving antenna beam forming using a corresponding algorithm. Among them, any antenna unit, corresponding feeder cable and coherent radio frequency transceiver together is called a link. By using weight getting from the up link receiving beam forming of each link in the down link transmitting beam forming, the entire functionality of smart antennas can be implemented, under symmetrical wave propagation conditions.
A primary aspect of modern wireless communication systems is mobile communication. Mobile communication works within a complex and variable environment (reference to ITU proposal M1225). Accordingly severe influences of time-varying and multipath propagation must be considered. The Chinese patent referenced above as well as many technical documents concerning beam forming algorithms of smart antennas conclude increased functionality will result with increased algorithm complexity. Nevertheless, under a mobile communication environment, beam forming must be completed in real time, and algorithm-completion time is at a microsecond level. As another limitation of modern microelectronic technology, digital signal processing (DSP) or application specific integrated circuits (ASIC) cannot implement highly complex real time processing within such short time periods. Faced with this conflict, within a mobile communication environment, simple and real time algorithms for smart antennas not only cannot solve the multipath propagation problem, but also cannot thoroughly solve system capacity problems of CDMA mobile communication systems.
Technologies such as the Rake receiver and Joint Detection or Multi User Detection have been widely studied for use in CDMA mobile communication systems in an attempt to solve the interference problems associated with multipath propagation. Nevertheless, neither the Rake receiver nor multiuser detection technology can be directly used in mobile communication systems with smart antennas. Multiuser detection technology processes the CDMA signals of multiple code channels, after channel estimation and matched filter, and all user data are solved at the same time using an inverse matrix. However smart antenna technology makes beam forming for each code channel separately, and so it is difficult to take advantage of the diversity provided by user multipath technology. Rake receiver technology composes user main multipath components, but it also destroys the phase relationship between antenna units of an antenna array. Another limitation of Rake receiver technology is that the user number is the same as the spread spectrum coefficient, which makes it impossible to work under full code channel circumstances.
There is a two-dimensional smart antenna technology, but it is in a research stage and its algorithm is immature and complex.
There is another method which processes multiuser detection after using smart antenna; but at this time as each code channel has been separated, processing must be separated for each code channel. As a result this technology not only cannot fully bring multiuser detection function into play, but it also greatly increases the complexity of baseband signal processing.